The present invention relates to a distortion compensating apparatus and, more particularly, to a distortion compensating apparatus for compensating for a distortion of a transmission power amplifier, using the correlation between a transmission signal and a feedback signal, and adjusting a timing of each element of the distortion compensating apparatus on the basis of a delay time obtained.
Frequency resources have become strained in recent years and greater reliance is being placed upon highly efficient digital transmission in radio communication. In a case where multivalued amplitude modulation is applied to radio communication, an important technique used on the transmission side is to linearize the amplification characteristic of a power amplifier to suppress a nonlinear distortion thereby and reduce the leakage power between adjacent channels. In a case where an amplifier having an inferior linearity is used and an attempt is made to improve the power efficiency, a technique for compensating for the distortion produced by this amplifier is essential.
FIG. 23 is a block diagram of an example of a transmitter in a conventional radio apparatus. A transmission signal generator 1 sends a group of serial digital data, and a serial/parallel (S/P) converter 2 alternately separates the group of digital data bit by bit into an in-phase component (I) signal and a quadrature component (Q) signal. A D/A converter 3 converts each of the I signals and Q signals into analog baseband signals and inputs the base band signals into a quadrature modulator 4. The quadrature modulator 4 multiplies the input I and Q signals (transmission baseband signals) by a reference carrier wave and signal phase-shifted from the reference carrier by 90xc2x0, respectively, adds the two products, thereby performing a quadrature conversion, and outputs the result. A frequency converter 5 mixes the signal subjected to quadrature modulation and a local oscillation signal to perform a frequency conversion, and a transmission power amplifier 6 amplifiers the power of the carrier wave output by the frequency converter 5 and radiates the amplified signal into space from an antenna 7.
In such a transmitter, the input/output characteristic (distortion function f(p) of the signal is a non-linear characteristic, as indicated by the dashed line in FIG. 24A. This nonlinear characteristic produces a nonlinear distortion, and a frequency spectrum centered about a transmission frequency f0 possesses side lobes, as indicated by the dashed line in FIG. 24B. This results in a leakage of the signal into the adjacent channels and causes interference on the adjacent channels. To prevent this, a Cartesian loop method, a polar loop method, etc. have been proposed as a technique for compensating a distortion of a feedback system, and a distortion of a power amplifier is suppressed.
FIG. 25 is a block diagram of a transmission apparatus having a function for digitally compensating for a nonlinear distortion by using a DSP (Digital Signal Processor). The group of digital data (transmission signals) sent from the transmission signal generator 1 is converted into I signals or Q signals by the S/P converter 2, and input into a distortion compensator 8 which is constituted by a DSP. The distortion compensator 8 as shown in the functional diagram of FIG. 26, is provided with a distortion compensation function memory 8a for storing the distortion compensation coefficient h(pi) (i=0xcx9c1023) which corresponds to the power level 0xcx9c1023 of a transmission signal, a pre-distortion unit 8b which applies a distortion compensation processing (pre-distortion) to the transmission signal by using the distortion compensation coefficient h(pi) which corresponds to the level of the transmission signal, and a distortion compensation coefficient arithmetic unit 8c which compares a transmission signal with a (feed back) signal demodulated by a later-described quadrature detector, and which calculates and updates the distortion compensation coefficient h(pi) by using the error.
The distortion compensator 8 applies a pre-distortion processing to the transmission signal by using the distortion compensation coefficient h(pi) which corresponds to the level of the transmission signal, and inputs the processed signal to the DA converter 3. The DA converter 3 converts the input I and Q signals into analog baseband signals and inputs the baseband signals into the quadrature modulator 4. The quadrature modulator 4 multiplies the input I and Q signals by a reference carrier wave and a signal phase-shifted from the reference carrier by 90xc2x0, respectively, adds the two products, thereby performing a quadrature conversion, and outputs the result. The frequency converter 5 mixes the signal subjected to quadrature modulation signal with a local oscillation signal to perform a frequency conversion, and the transmission power amplifier 6 amplifiers the power of the carrier wave output from the frequency converter 5 and radiates the amplified signal into space from the antenna 7. A part of the transmission signals are input into a frequency converter 10 via a directional coupler 9. The frequency converter 10 converts the frequency of the signals and inputs them into a quadrature detector 11. The quadrature detector 11 multiplies each of the input signal by the reference carrier wave and by the signal phase-shifted from the reference carrier by 90xc2x0, thereby performing quadrature detection and reproducing the baseband I, Q signals from the transmission side, and inputs these signals into an AD converter 12. The AD converter 12 converts the input I and Q signals into digital signals and inputs them into the distortion compensator 8. The distortion compensator 8 compares the transmission signal before a distortion compensation processing with the feedback signal which is demodulated by the quadrature detector 11 by an adaptive algorithm using the LMS (Least Means Square) method, and calculates and updates the distortion compensation coefficient h(pi) by using an error between the transmission signal and the feedback signal. The transmission signal to be sent next is then subjected to a pre-distortion processing using the updated distortion compensation coefficient h(pi), and the processed signal is output. The above-described operation is repeated thenceforce, thereby suppressing the nonlinear distortion of the transmission power amplifier 6 and reducing the power leaked to the adjacent channels.
FIG. 27 is an explanatory view of the distortion compensation processing based on an adaptive algorithm using an LMS method. The reference numeral 21a denotes a multiplier (pre-distortion unit) for multiplying a transmission signal (input baseband signal) x(t) by a distortion compensation coefficient hn(p), 21b a DA converter for converting the transmission signal subjected to a distortion compensation processing into an analog signal, 21c a device (transmission power amplifier) in which a distortion generates and which has a distortion function f(p), 21d a feedback system for feeding back a signal y(t) output from the transmission power amplifier, 21e an AD converter for converting a feedback signal into a digital signal, and 21f a distortion compensation coefficient memory for storing the distortion compensation coefficient hn(p) in correspondence with the power p (=|x(t)|2) of the transmission signal x(t). The distortion compensation coefficient memory 21f updates the distortion compensation coefficient hn(p) by a distortion compensation coefficient hn+1(p) which is obtained by an LMS algorithm.
The reference numeral 21g denotes an arithmetic unit for calculating the power p (=|x(t)|2) of the transmission signal x(t) and outputs the power as a reading address, 21h a delay circuit for generating a writing address of the distortion compensation coefficient memory 21f. It takes a predetermined time to obtain a new distortion compensation coefficient hn+1(p) since the distortion compensation coefficient hn(p) at the address which is indicated by the power p of the transmission signal x(t) is read. Therefore, the delay circuit 21h delays the generation of a writing address by the predetermined time, so as to be able to update the old distortion compensation coefficient hn(p) by the new distortion compensation coefficient hn+1(p). The reference numeral 21h denotes a delay time adjuster.
The reference numeral 21j denotes a distortion compensation coefficient arithmetic unit for calculating by an LMS adaptive algorithm and updating a distortion compensation coefficient in such a manner that the error is zero. In the distortion compensation coefficient arithmetic unit 21j, the reference numeral 21j-1 denotes a subtracter for outputting the difference e(t) between a transmission signal x(t) before a distortion compensation processing and a feedback signal y(t), 21j-2 a delay circuit for adjusting the timings of a transmission signal x(t) and a feedback signal y(t), 21j-2xe2x80x2 a delay time adjuster for adjusting the delay time of the delay circuit 21j-2, 21j-3 a multiplier for multiplying the error e(t) by a step size parameter xcexc( less than 1), 21j-4 a conjugate complex signal outputting unit for outputting a conjugate complex signal y*(t), 21j-5 a multiplier for multiplying hn(p) by y*(t), 21j-6 a multiplier for multiplying xcexce(t) by u*(t), 21j-7 a delay circuit for adjusting the timing for outputting the distortion compensation coefficient hn(p), 21j-8 an adder for adding the distortion compensation coefficient hn(p) and xcexce(t)u*(t), and 21j-9 a delay time adjuster for adjusting the delay time of the delay circuit 21j-7.
The distortion compensation coefficient arithmetic unit 21j having the above-described structure performs the following operations:
hn+1(p)=hn(p)+xcexce(t)u*(t)
e(t)=x(t)xe2x88x92y(t)
y(t)=hn(p)x(t)f(p)
u(t)=x(t)f(p)=h*n(p)y(t)
P=|x(t)|2
wherein the symbols x, y, f, h, u and e represent complex numbers and *a conjugate complex number. By executing the arithmetic processing shown above, the distortion compensation coefficient hn(p) is updated, and eventually converges on the optimum distortion compensation coefficient, which compensates for the distortion of the transmission power amplifier.
In a conventional distortion compensating method, however, there is one problem. If it is assumed that the delay time of the transmission power amplifier 21c is D0, and the delay time of the feedback system 21d is D1, it is necessary to set the delay time D in each of the delay circuits 21h, 21j-2 and 21j-7 in such a manner as to satisfy the following formula:
D=D0+D1
However, since the devices such as the transmission power amplifier 21c and the feedback system 21d have individual differences, the total delay time D varies. For this reason, it is conventionally necessary to manually adjust the delay times of the delay circuits 21h, 21j-2 and 21j-7 one by one by using a delay time adjusting switches 21i, 21j-2xe2x80x2 and 21j-9. In spite of such a troublesome delay time adjusting operation, high precision cannot be expected.
Accordingly, it is an object of the present invention to eliminate the above-described problems in the related art, and to enable a delay time to be automatically measured and the measured delay time to be set in a delay circuit.
It is another object of the present invention to enable a delay time to be measured with high precision and the measured delay time to be set in a delay circuit.
To achieve these objects, in a first aspect of the present invention, there is provided a distortion compensating apparatus comprising: (1) a pre-distortion unit for applying a distortion compensation processing to a transmission signal by using a distortion compensation coefficient, (2) a distortion compensation coefficient arithmetic unit for calculating a distortion compensation coefficient on the basis of the transmission signal before the distortion compensation processing and a feedback signal sent from an output side of a transmission power amplifier, (3) a distortion compensation coefficient memory for storing the calculated distortion compensation coefficient in correspondence with the transmission signal, (4) a delay time decision unit for calculating the correlation between the transmission signal and the feedback signal while varying the phase difference between both signals, and deciding the total delay time caused in the transmission power amplifier and a feedback loop on the basis of the phase difference in which the correlation is the maximum, and (5) a delay unit for delaying the transmission signal by the decided delay time, and inputting the delayed transmission signal into the distortion compensation coefficient arithmetic unit.
In a second aspect of the present invention, there is provided a distortion compensating apparatus comprising: (1) a pre-distortion unit for applying a distortion compensation processing to a transmission signal by using a distortion compensation coefficient at sampling period of the transmission signal, (2) a distortion compensation coefficient arithmetic unit for calculating a distortion compensation coefficient on the basis of the transmission signal before the distortion compensation processing and a feedback signal sent from the output side of a transmission power amplifier, (3) a distortion compensation coefficient memory for storing the calculated distortion compensation coefficient in correspondence with the transmission signal, (4) a feedback signal delay unit for controlling the amount of delay at interval of time shorter than the sampling period of the transmission signal, (5) a correlation arithmetic unit for calculating the correlation between the transmission signal and the feedback signal while sequentially varying the time difference between both signals at interval of the sampling period, (6) a control unit for obtaining the amount of delay in the feedback signal delay unit and the time difference in which the correlation is the maximum, and deciding the total delay time caused in the transmission power amplifier and a feedback loop on the basis of the amount of delay and the time difference, and (7) a transmission signal delay unit for delaying the transmission signal by the total delay time, and inputting the delayed transmission signal into the distortion compensation coefficient arithmetic unit.
To state this concretely, the control unit (1) obtains the time difference in the unit of sampling period in which the correlation is the maximum while the amount of delay in the feedback signal delay unit is held constant, and then (2) fixes the time difference between the transmission signal and the feedback signal in the correlator at the delay time obtained, and adjusts the amount of delay which is smaller than the sampling period in the feedback signal delay unit in such a manner that the correlation is the maximum.
Alternatively, the control unit (1) obtains the time difference in the sampling period in which the correlation is the maximum while the amount of delay in the feedback signal unit is held constant, (2) sets the time difference as the delay time in the transmission signal delay unit, thereby canceling the delay time in the unit of the sampling period, (3) fixes the time difference between a transmission signal and a feedback signal in the correlator at 0, and (4) adjusts the delay time which is smaller than the sampling period in the feedback signal delay unit in such a manner that the correlation is the maximum.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.